1. Field of the Invention
The present invention relates to an ignition timing apparatus of an internal combustion engine.
2. Description of the Related Art
Knocking is usually detected by use of a knocking sensor which generates an output voltage proportional to the strength of the vibration of a cylinder block of an engine. It is judged if knocking has occurred by detecting the strength of the vibration of the cylinder block by the knocking sensor in the range of crank angle where knocking may occur. Knocking sensors, however, also pick up vibration other than the vibration of the cylinder block generated by knocking, so it is necessary to discriminate the vibration generated by knocking from other vibration. Usually, therefore, the mean value of the output voltages of the knocking sensor or the mean value of the peak values of output voltage of the knocking sensor is deemed to be the strength of the vibration generated due to reasons other than knocking. This is used as the background noise value An. This background noise value An is multiplied by a certain coefficient K (K&gt;1.0). The result is used as the knocking judgement value An.multidot.K. When the output voltage of the knocking sensor exceeds the knocking judgement value An.multidot.K, it is judged that knocking has occurred. When it is judged that knocking has occurred, a common ignition timing for all cylinders is delayed by a certain predetermined angle. Note that the background noise value An differs for each cylinder, so different background noise values are given to the cylinders.
As mentioned above, however, the background noise value An is the mean value of the output voltages of the knocking sensor or the mean value of the peak values of the output voltages of the knocking sensor, so if knocking occurs a number of times in a certain specific cylinder, for example, the no. 1 cylinder, the background noise value An of the no. 1 cylinder gradually becomes larger. If the background noise value An becomes gradually larger in this way, however, even if knocking actually occurs in the no. 1 cylinder, it is judged that no knocking has occurred and therefore no delay is given to the ignition timing, so there is the problem that knocking will continue.
To resolve the above problem, it has been considered to find the frequency of occurrence of knocking for each cylinder and to greatly delay the ignition timing when the largest frequency among these exceeds a certain value. It takes time, however, to find the frequency of occurrence of knocking in this way and therefore the delay action on the ignition timing is too slow during transitional engine operations, so the problem of frequent knocking remains.